Lineweaver and Chopra reviewed current research examining environments where life is found on Earth and the environments thought to exist on other planets.

Since the first discovery of a planet orbiting another star was made in 1995, the number of exoplanets has skyrocketed to more than 750. While a small handful of these planets are known to be 'Earth-like', astronomers are a long way from knowing whether they can sustain life.

"Determining whether these planets are habitable has become the new holy grail of astronomy," says Lineweaver. "It's probably one of the biggest, most confusing, and important issues that planetary scientists are going to have to deal with in the next 10 to 20 years."

Lineweaver says one of the reasons why humans should search for habitable planets is to place future human colonies. He dismisses the idea that humans should stay on Earth, comparing it to the attitude of some towards Columbus' proposed trip across the Atlantic Ocean.

"It's a bit like the Europeans in 1450 saying 'Hey what does it matter whether we go exploring the rest of the world?'"

Comparing planets

Lineweaver and Chopra's review of the literature found that the presence of water and a temperature range of between -20°C and 122°C are the two most important parameters for harbouring life.

"Over the past few decades our exploration of the Earth has turned up life in all kinds of weird environments where we didn't think life could be in, and we're finding all types of extraterrestrial environments that we didn't know about before," says Lineweaver. "As these two groups expand they start to overlap in big ways, and that's where habitable planets will be found."

The report also raises the possibility of habitable planets that don't contain life. They argue that the conditions for life to form, called the abiogensis habitable zone, are much narrower than the conditions needed for life to survive.

"Life, by managing its own environment, makes a planet habitable. It has produced adaptive features as a result of Darwinian evolution to live in colder and warmer environments," says Lineweaver. "It's kind of like an adult can live in a higher range of temperatures than a baby can."

Lineweaver believes observation programs such as the Kepler telescope, which has been extended to at least 2015, will continue to discover more Earth-like planets.

"The next step will be to develop a satellite that can look at the atmospheres of these planets, which will be able to give us some information about whether there is life there or not," he says.

If we find an Earth-like planet, Lineweaver says the next step is to send an interstellar probe to explore it.